Former House Speaker Newt Gingrich visits Kanzi, a 25-year-old Bonobo, at Great Ape Trust of Iowa in 2005. (Steve Pope/Associated Press)

In November, Mischa Fisher took to this space to criticize the notion that Republicans are the anti-science party. Plenty of Democrats hold views that contradict empirically established facts, and Republican skepticism is overblown, he wrote.

And yet ...

The Pew Research Center released new numbers Monday on how Americans view evolution. (The question was asked in a way to include those who believe God or a supreme being guided the process.) About six in 10 accept it, the poll found, but the partisan divide is wide:

It's not surprising that Republicans are less likely to believe in evolution that Democrats are; while the numbers vary from survey to survey, there has been a consistent gap. Republicans are also less likely to believe that the earth's climate is warming, or, if they accept that it is, to believe that the change is caused by human activity. But belief in climate change is actually on the uptick, among both Democrats and Republicans, having reached a nadir in 2009. (Some academics believe the recession helped to depress belief in warming, as people's worries about their immediate livelihood trumped longer-term concerns.)

What's surprising in the new Pew evolution numbers is the trend—a more than 10-point drop in belief among Republicans. What explains it?

Pew doesn't speculate but remarks on the confusing result:

Differences in the racial and ethnic composition of Democrats and Republicans or differences in their levels of religious commitment do not wholly explain partisan differences in beliefs about evolution. Indeed, the partisan differences remain even when taking these other characteristics into account.

One possibility is that respondents who identified as Republican and believed in evolution in 2009 are no longer identifying as Republicans. Fewer scientists, for example, are reportedly identifying with the GOP, and the overall trend is for fewer Americans to call themselves Republicans. But both Gallup and separate polling from Pew found approximately the same party ID in 2009 and 2013.

Another is that the rise of "intelligent design" education has helped to swing younger Americans against evolution. Yet the age breakdown remains similar in 2009 and 2013, with respondents ages 18 to 29 most likely to believe in evolution.

What does that leave? Maybe the gap represents an emotional response by Republicans to being out of power. Among others, Chris Mooney has argued that beliefs on politically contentious topics are often more rooted in opposition to perceived attacks than anything else—an instance of "motivated reasoning." Given that Democrats have controlled the White House and Senate since 2009, this could be backlash to the political climate, though it will be hard to tell until Republicans control Washington again.

Of course, motivated reasoning might help explain why many Democrats also believe in evolution.